1. Field of the Invention
This invention relates to a magnetic disk drive such as a hard disk drive or the like, and more particularly to a moving coil type rotary actuator suitable for used on a magnetic disk drive for driving a magnetic head arm in accessing to a specified track on a disk.
2. Description of the Prior Art
In a magnetic disk drive, in order to gain access to a specified track on a disk, a magnetic head arm is driven by a moving coil type rotary actuator which is arranged, for example, as shown in FIG. 1. For tracking a specific track by a magnetic head 51 on one end of a magnetic head arm 52, a moving coil 53 is provided at the other end of the arm which is rotatably supported on a rotational shaft 54.
Located on the opposite sides of the moving coil 53 are first and second yorks 55 and 56 which have first and second pairs of permanent magnets 57 and 58 and 59 and 60 (each pair being formed by the adjacently positioned magnets) securely fixed thereon. The moving coil 53 is rotatably supported between the inner opposing surfaces of the first and second yorks 55 and 56 with the first and second pairs of permanent magnets 57 and 58 and 59 and 60, respectively.
As shown in FIG. 2, the moving coil 53 is constituted by first and second effective coil portions 61 and 62, first and second ineffective coil portions 63 and 64, and R (round) portions 65 to 68 connecting the first and second effective coil portions 61 and 62 with the first and second ineffective coil portions 63 and 64.
The first and second pairs of permanent magnets 57 and 58 and 59 and 60 are each arranged to have an inverse polarity on the side of the first and second effective coil portions 61 and 62 (or on the side of the magnetic gap) relative to the other one of the same pair (the magnets fixed on the same yoke) and relative to the opposing one of the other pair (the magnets fixed on the other yoke) as shown in FIG. 3. Namely, for example, facing the effective coil portions 61 and 62, the permanent magnet 57 has the N pole, the permanent magnet 58 the S pole, the permanent magnet 59 the S pole, and the permanent magnet 60 the N pole.
In operation of the prior art moving coil actuator of the above-described arrangement, if current I is passed through the effective coil portions 61 and 62 of the moving coil 53 in the direction shown in FIG. 3, a propulsive force urges the moving coil 53 in the direction of arrow A according to Flemming's left-hand rule, turning the arm 52 and the magnetic head 51 for access to a specified track. Conduction of current I in the opposite direction generates a propulsive force in the opposite direction.
Recently, the moving coil type actuators for magnetic disk drives have been required to have a greater propulsive force in order to meet the strong demands for reductions in size and thickness. In this connection, magnets of rare earth cobalt, like samarium, with a large energy product, are resorted to as permanent magnets in most cases. However, since rare earth cobalt magnets are very expensive, attempts have been made to enhance the efficiency in usage of such permanent magnets, reducing the weights of the permanent magnets by eliminating them from those portions which do not effectively contribute to generation of the propulsive force. The first and second effective coil portions 61 and 62 contribute to the propulsive force while the first and second ineffective coil portions 63 and 64 do not. The R portions 65 to 68 contribute to the propulsive force only at a rate smaller than 50% in terms of their length, and obviously not at a rate of 100%. Therefore, it is often the case to employ an arrangement in which the first and second pairs of permanent magnets 57 and 58 and 59 and 60 have widths which are equal to the length of the effective coil portions 61 and 62 in width in a direction perpendicular to the direction of the propulsive force on the moving coil 53.
FIG. 4 illustrates another prior art moving coil type rotary actuator which has only the second pair of permanent magnets 59 and 60 fixed ont he second yoke 56.
In case of the conventional moving coil type rotary actuators as shown in FIGS. 1 through 4, which have the first pair of permanent magnets 57 and 58 and/or the second pair of permanent magnets 59 and 60 fixed side by side on the first yoke 55 and/or the second yoke 56 in such a manner as to have inverse polarity relative to the other one, there arises a problem of diminution of main flux .phi..sub.M due to leakage flux .phi..sub.R of minor loop as shown in FIG. 3.
Consequently, the propulsive force of the actuator becomes weaker, resulting in a slower access to a desired track. Therefore, it becomes necessary to provide larger permanent magnets to compensate for the diminution of the main flux .phi..sub.M, which makes it difficult to reduce the size of the actuator as a whole and, due to expensiveness of the magnets, invites a substantial increase in cost.